Plastic molded article or product and method and apparatus for producing the same by injection molding

ABSTRACT

An apparatus for producing a plastic molded article or product for use in an optical device is a mold assembly. The mold assembly includes a pair of molds having an inner mold surface forming a cavity with a preselected volume into which a molten molding material is injected via a gate or sprue to fill the cavity and thereby form a molded product having a sink surface and at least one mirror surface after the molten molding material has cooled. The inner mold surface includes at least one transfer surface portion against which the at least one mirror surface of the molded product is formed, and at least one non-transfer surface portion against which the sink surface portion of the molded product is formed. The mold assembly also includes at least one vent hole, at least one bore in communication with the vent hole so that a preselected air pressure is applied to the molten molding material in the cavity via the bore and the vent hole, and at least one exhaust hole adjacent to, but spaced from the vent hole. Thus, only a part of the molded article or product expected to sink is surely caused to sink while, e.g., a mirror surface is surely transferred to a desired part of the molded article or product.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens, mirror, prism, or similarplastic molded article or product produced by injection molding andincluded in an optical device, e.g., a copier, laser printer, facsimileapparatus, or similar image forming apparatus, and a method andapparatus for producing the same. More particularly, the presentinvention is concerned with a plastic molded article or product having,e.g., mirror surfaces and a fine undulation pattern transferred theretowith high accuracy by injection molding, and a method and apparatus forproducing the same.

2. Discussion of Background

For injection molding, it is a common practice to use a mold assemblyincluding an inner mold surface forming a cavity having a preselectedvolume, a transfer surface formed on the inner mold surface fortransferring a mirror surface to a molded article or product, and a gateor sprue open at the inner mold surface and having a preselected openingarea. Molten resin is injected into the cavity via the gate or sprue andthen cooled. The resulting molded article or product is taken out byopening the mold assembly. While such a molded article or product,particularly a mirror, lens, prism, or similar optical element, isrequired to have an accurate mirror surface and a uniform refractiveindex, the mirror surface needing a high surface accuracy is caused tosink because the molten resin contracts at the time of solidification.

Injecting molding methods for solving the above-described problem aretaught in, e.g., Japanese Patent Laid-Open Publication Nos. 3-128218,8-234005, 3-151218, and 3-281213 (hereinafter “Prior Art 1”). In PriorArt 1, either a non-transfer surface, or a mold surface which faces atransfer surface, is formed with, e.g., a roughened mirror surface or asurface treated so as to have a lower wettability, or else use is madeof a porous material. Injection is stopped just before a cavity isfilled up with molten resin. Then, the molten resin is solidified bycooling without any dwelling. As a result, the roughened surface iscaused to sink due to a difference in adhering force between the moltenresin, the transfer surface, and the roughened surface. This preventsthe mirror surface of the molded article or product from sinking.Alternatively, an overflow portion for receiving excess molten resin islocated outside of the cavity. When the overflow portion begins to befilled, injection is stopped. Then, the molten resin is solidified bycooling without any dwelling. This also allows the roughened surface tosink due to a difference in adhering force between the resin, thetransfer surface, and the roughened surface.

An injection molding method disclosed in Japanese Patent Laid-OpenPublication No. 2-175115 (hereinafter “Prior Art 2”) injects moltenresin into a cavity in which a porous member, which communicates with acompressed gas such as air, is provided on an inner mold surface so asto contact the non-transfer surface of a molded article or product.While dwelling and cooling are under way after the injection of themolten resin, air is fed to the non-transfer surface of the moldedarticle or product via the porous member. With this method, a side of acylindrical thin lens (i.e., the molded article or product) may becaused to sink.

Japanese Patent Laid-Open Publication No. 6-304973 (hereinafter “PriorArt 3”) proposes an injection molding method in which a non-transfersurface communicates with the outside air via a vent hole. During aninterval between the beginning and the end of the injection of moltenresin into a cavity, a pressure difference is generated between thetransfer surface and the non-transfer surface of the resin. As a result,the non-transfer surface of the resin is caused to sink. Specifically,air is brought into contact with the molten resin, on a surface otherthan the mirror surface of the molded article or product transferredfrom the transfer surface of the mold assembly, via the vent hole and abore communicating therewith, so that the cooling speed of the moltenresin is lowered. At the same time, a preselected air pressure is fed tothe vent hole in order to generate a preselected pressure differencebetween the mirror surface of the molded article or product and the venthole. This allows only the portion of the resin facing the vent hole tosink, i.e., prevents the mirror surface of the molded article or productfrom sinking. In addition, because only the portion of the resin facingthe vent hole sinks, a molded article or product can be produced bysimple control over the amount of the molten resin to be injected intothe cavity and without any strain being generated in the resin. Theresulting molded article or product is therefore free from an internalstrain and is provided with an accurate mirror surface.

Prior Art 3 further teaches that the vent hole may communicate with acompressor so as to apply a preselected air pressure to the portion ofthe resin facing the vent hole. With this configuration, it is possibleto generate any desired pressure difference between the mirror surfaceof the molded article or product and the portion of the resin facing thevent hole, thereby causing the portion of the resin facing the vent holeto sink. In addition, the pressure difference is readily adjustable inorder to further enhance the accuracy of the mirror surface without anyinternal strain.

Japanese Patent Laid-Open Publication No. 6-315961 (hereinafter “PriorArt 4”) teaches an injection molding method causing the non-transfersurface of resin to sink. In accordance with this method, the transfersurface of a mold assembly is heated to and held at a high temperature.The transfer surface side of the resin is heated to a high temperatureuntil the injection of molten resin into a cavity ends.

However, the above-described injection molding methods of Prior Art 1,Prior Art 2, Prior Art 3, and Prior Art 4 have some drawbacks. Moreparticularly, Prior Art 1, which relies on any one of the roughenedsurface, surface treatment, or porous material, results in an expensivemold assembly. Moreover, stopping the injection just before the cavityis filled up with the molten resin is extremely difficult. Should thecorrect timing for stopping the injection of molten resin not berealized, the relationship in the adhering forces between the transfersurface and the roughened surface would be inverted and would therebycause the mirror surface of the molded article or product to sink orelse result in a shortage of resin. In addition, because sinking cannotbe provided with directionality and because setting the moldingconditions is difficult, the configuration of the molded article orproduct is critically limited. It is more preferable that the injectionof the molten resin be stoppable at any time within a broader range oftimes. However, in this case, the overflow portion, which is formedintegrally with the molded article or product, must be removed by anextra step. Moreover, if the area of the opening of the gate or spruefor feeding the molten resin to the overflow portion is excessivelysmall, the relationship in adhering force between the transfer surfaceand the roughened surface would also be inverted and would thereby causethe mirror surface of the molded article or product to sink. Also, therewould not be enough of the molten resin.

Prior Art 1 can be implemented as a mirror or similar optical elementneeding a single mirror surface, because it roughens the mold surfacefacing the transfer surface. However, Prior Art 1 cannot produce a lens,prism, or similar optical element, because the number and positions ofthe mirror surfaces are limited. In addition, the relationship inadhering force is inverted and causes the mirror surface of the moldedarticle or product to sink, depending on the material constituting thetransfer surface and roughened surface and the kind of resin used.

Prior Art 2 increases the cost of the mold assembly due to the porousmember and provides more sophisticated control over the configuration ofthe porous member. Specifically, if the effect of the porous member isexcessive, it not only admits the molten molding material thereinto, byalso obstructs the parting of the molded article or product from themold assembly. This is particularly true when the porous portion of theporous member extends inwardly over the wall of the mold assembly.Further, because the compressed gas is fed to the non-transfer surfaceof the molded article or product via the porous member during thepreviously stated interval, a pressure difference is maintained betweenthe non-transfer surface and the transfer surface of the molten resinduring cooling. As a result, the internal strain remains in theresulting molded article or product after the opening of the moldassembly. The residual pressure not only lowers the accuracy of thetransfer surface, but also causes the entire molded article or productto deform.

Prior Art 3 generates a pressure difference between the transfer surfaceand the non-transfer surface of the resin during the interval mentionedearlier. This also brings about the problem stated above in relation toPrior Art 2. Prior Art 4 maintains the transfer surface of the moldassembly at high temperature and heats the transfer surface side of theresin to a high temperature during the previously mentioned interval.This is also undesirable in the above-described respect.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a plasticmolded article or product, wherein only a desired portion thereof sinks,wherein a mirror surface is provided in another desired portion thereof,and wherein a method and an apparatus for producing the same byinjection molding are further provided.

It is another object of the present invention provided an inexpensiveand least deformable plastic molded article or product, wherein only anon-transfer surface thereof sinks so that the molded article or producthas a highly accurate transfer surface.

In accordance with the present invention, in a molded article or productproduced by an injection mold assembly having a pair of molds includingan inner mold surface forming a cavity having a preselected volume, atleast one transfer surface for transferring a mirror surface to themolded article or product from the inner mold surface, and a gate orsprue for filling the cavity with a molten molding material byinjection, and by injecting the molten molding material into the cavityvia the gate or sprue and then cooling the molten molding material, theinjection mold assembly includes at least one vent hole having apreselected opening area, and at least one bore communicating with thevent hole for applying a preselected air pressure to the molded articleor product. A step portion is formed on the inner mold surface betweenthe vent hole and the transfer surface.

Also, in accordance with the present invention, in an injection moldingmethod for producing a molded article or product by using a moldassembly having a pair of molds including an inner mold surface forminga cavity having a preselected volume, at least one transfer surface fortransferring a mirror surface to the molded article or product from theinner mold surface, and a gate or sprue for filling the cavity with amolten molding material by injection, and by injecting the moltenmolding material into the cavity via the gate or sprue and then coolingthe molten molding material, the inner mold surface is formed with,outside of the transfer surface, with at least one vent hole having apreselected opening area and at least one bore communicating with thevent hole for applying a preselected air pressure to the molten moldingmaterial. The air pressure is continuously generated via the vent holeeven after the pressure of the molten molding material in the cavity hasdropped to zero.

Further, in accordance with the present invention, a mold assembly has apair of molds including an inner mold surface forming a cavity having apreselected volume, at least one transfer surface for transferring amirror surface to a molded article or product from the inner moldsurface of the mold assembly, and a gate or sprue for filling the cavitywith a molten molding material by injection, and injects the moltenmolding material into the cavity via the gate or sprue and then coolsthe molten molding material. The inner mold surface is formed with,outside of the transfer surface, at least one vent hole having apreselected opening area and at least one bore communicating with thevent hole for applying a preselected air pressure to the molten moldingmaterial, and at least one exhaust hole located at a position adjoiningthe vent hole, but not facing the transfer surface.

Moreover, in accordance with the present invention, a method ofproducing a plastic molded article or product begins with the step ofpreparing a mold assembly including at least one transfer surface and atleast one non-transfer surface formed on a surface other than thetransfer surface. The transfer surface and non-transfer surface forms atleast one cavity. Molten resin, heated to a temperature above asoftening point thereof, is injected into the cavity. A resin pressureis caused to act on the transfer surface to thereby cause the moltenresin to adhere to the transfer surface, and then the molten resin iscooled to a temperature below the softening point. The mold assembly isopened in order to allow the resulting molded article or product to betaken out. The temperature of at least one non-transfer surface of theresin is lowered below the temperature of the resin on the transfersurface during an interval between the beginning and the end ofinjection of the molten resin into the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription in accordance with the accompanying drawings, in which:

FIG. 1 is a fragmentary view showing a conventional injection moldassembly;

FIG. 2 is a plan view showing a specific vent hole formed in the innermold surface of a sink insert included in the mold assembly of FIG. 1;

FIG. 3A is a perspective view showing a specific molded article orproduct produced by the mold assembly of FIG. 1;

FIG. 3B is a side elevational view of the molded article or product ofFIG. 3A;

FIG. 3C is a cross-sectional view taken along line 3 c—3 c of FIG. 3B;

FIG. 4A is a side elevational view showing a specific molded article orproduct produced by injection molding with an air stream flowing towardthe reference surface of a cavity;

FIG. 4B is a cross-sectional view taken along line 4 b—4 b of FIG. 4A;

FIG. 5A is a top plan view showing a specific molded article or productproduced by injection molding and caused to sink as far as its mirrorsurface portion;

FIG. 5B is a side elevational view of the molded article or productshown in FIG. 5A;

FIG. 5C is a cross-sectional view taken along line 5 c—5 c of FIG. 5B;

FIG. 6 shows a relationship between the molded article or productproduced by the mold assembly of FIG. 1 and the position of a vent hole;

FIG. 7A is a perspective view showing a molded article or product and aninjection mold assembly representative of a first embodiment of thepresent invention;

FIG. 7B is a fragmentary cross-sectional view of the mold assembly shownin FIG. 7A;

FIG. 8 is a perspective view of a molded article or productrepresentative of a second embodiment of the present invention;

FIG. 9A is a perspective view of a molded article or productrepresentative of the third embodiment of the present invention;

FIG. 9B is a cross-sectional view of the third embodiment taken alongplane 9 b—9 b—9 b of FIG. 9A;

FIG. 9C is a cross-sectional view of the third embodiment taken alongplane 9 c—9 c—9 c of FIG. 9A;

FIGS. 10A and 10B are cross-sectional views of a molded article orproduct each showing a particular configuration of steps included in thethird embodiment, wherein FIG. 10A shows steps protruding from the sinksurface of the molded article or product and FIG. 10B shows stepsextending into the sink surface of the molded article or product;

FIG. 11A is a perspective view of a molded article or productrepresentative of a fourth embodiment of the present invention;

FIG. 11B is a cross-sectional view taken along plane 11 b—11 b—11 b ofFIG. 11A;

FIG. 12A is a perspective view of a molded article or productrepresentative of a fifth embodiment of the present invention;

FIG. 12B is a cross-sectional view taken along plane 12 b—12 b—12 b ofFIG. 12A;

FIG. 13A is a perspective view of a molded article or productrepresentative of a sixth embodiment of the present invention;

FIG. 13B is a cross-sectional view taken along plane 13 b—13 b—13 b ofFIG. 13A;

FIG. 14A is a perspective view of a molded article or productrepresentative of a seventh embodiment of the present invention;

FIG. 14B is a cross-sectional view taken along plane 14 b—14 b—14 b ofFIG. 14A;

FIG. 15 is a cross-sectional view of a molded article or productrepresentative of an eighth embodiment of the present invention andincluding tapered steps;

FIGS. 16A and 16B are cross-sectional views each showing a particularconfiguration of a molded article or product representative of a ninthembodiment of the present invention;

FIGS. 17A and 17B are graphs showing the variation of the internalpressure of molten resin existing in a cavity occurring from thebeginning to the end of cooling of the molten resin, and a timing forswitching an air pressure fed via a vent hole, respectively;

FIG. 18 is a cross-sectional view of an injection mold assemblyrepresentative of an eleventh embodiment of the present invention;

FIG. 19 is a perspective view showing a positional relationship betweena vent hole and an exhaust hole included in the eleventh embodiment;

FIG. 20 is a cross-sectional view of an injection mold assemblyrepresentative of a twelfth embodiment of the present invention;

FIG. 21 is a perspective view showing the position of an exhaust holeformed in an injection mold assembly representative of a thirteenthembodiment of the present invention;

FIG. 22 is a perspective view showing a modification of the thirteenthembodiment;

FIG. 23A is a perspective view of a plastic molded article or productrepresentative of a fifteenth embodiment of the present invention;

FIG. 23B is a side elevational view showing a sinking region occurringin the fifteenth embodiment;

FIG. 23C is a cross-sectional view taken along line 23 c—23 c of FIG.23B;

FIG. 24A is a cross-sectional view showing one-half of the fifteenthembodiment;

FIG. 24B is a perspective view showing cavity inserts included in thefifteenth embodiment;

FIG. 25A is a cross-sectional view taken along plane 25 a—25 a—25 a ofFIG. 24B;

FIG. 25B is a cross-sectional view taken along plane 25 b—25 b—25 b ofFIG. 24B;

FIG. 26 is a cross-sectional view showing a method and an apparatus forproducing a plastic molded article or product representative of asixteenth embodiment of the present invention;

FIGS. 27A-27D show a specific procedure available with the sixteenthembodiment;

FIGS. 28A-28D show another specific procedure available with thesixteenth embodiment;

FIG. 29 is a top plan view showing a method and an apparatus forproducing a plastic molded article or product representative of aseventeenth embodiment of the present invention;

FIGS. 30A and 30B show another specific procedure available with theseventeenth embodiment;

FIG. 31 is a top plan view showing a method and an apparatus forproducing a plastic molded article or product representative of aneighteenth embodiment of the present invention;

FIG. 32A is a perspective view showing mirror, reference, and sinkinserts included in the eighteenth embodiment;

FIG. 32B is a cross-sectional view taken along plane 32 b—32 b—32 b ofFIG. 32A;

FIG. 32C is a cross-sectional view taken along plane 32 c—32 c—32 c ofFIG. 32A;

FIGS. 33A and 33B show another specific procedure available with theeighteenth embodiment;

FIGS. 34A-34C show a procedure following the procedure of FIG. 32B; and

FIG. 35 shows a modification of the eighteenth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To better understand the present invention, brief reference will be madeto the injection molding method taught in Prior Art 3 mentioned earlier.As shown in FIG. 1, a mold assembly 10 is made up of a stationary orupper mold 11 and a movable or lower mold 12, a pair of mirror pieces orinserts 13 and 14, a reference insert 15, and a sink insert 16 togetherforming a cavity 17 having a preselected volume. Each of the mirrorpieces or inserts 13 and 14 has a transfer or mold surface 13 a and 14a, respectively, for transferring a mirror surface 22 and 23 to a moldedarticle or product 21 (see FIG. 3A). The reference insert 15 has a moldsurface or transfer surface 15 a for transferring a reference surface orC surface 25 to a molded article or product 21 (see FIG. 3A). The sinkinsert 16 has a mold surface or a sink surface 16 a for causing a sinksurface or B surface of a molded article or product 21 (see FIG. 3A) tosink. Molten resin or similar molten molding material 20 is injectedinto the cavity 17 via a gate or sprue (not shown) similar to the one(i.e., gate or sprue 63) shown in FIGS. 25B, 27A-D, 30A-B, 32C, 33A-B,and 34A-C. The mold surface or sink surface 16 a of the sink insert 16is formed with a vent hole 18 having a preselected opening area, and abore communicating with the vent hole 18. Air 9, under preselectedpressure, is fed to the molten resin or molten molding material 20 viathe bore 19 and vent hole 18.

FIG. 2 shows a specific configuration of the vent holes 18 formed in thesink inset 16 and extending to the mold surface 16 a. To feed air to themolten molding material 20, use may be made of natural draft utilizing apressure difference between the mirror portion of the molten moldingmaterial 20 and the vent hole portion, or forced draft generating adesired pressure difference between the two portions with a compressor(not shown) communicating with the vent hole 18.

In the mold assembly 10, the vent hole 18 is positioned at the side of amolded article or product which is expected to sink. When air 9 is fedto the cavity 17 via the bore 19 and vent hole 18, sinking successfullyoccurs in the expected surface (i.e., sink surface 24) of the moldedarticle or product. In addition, the transfer surfaces 13 a and 14 a ofthe mirror pieces or inserts 13 and 14, respectively, are desirablytransferred to the molded article or product. The molded article orproduct therefore suffers from a minimum of internal strain.

A specific molded article or product 21, produced by the mold assembly10, is shown in FIGS. 3A-3C. As shown, the molded article or product isimplemented as a rectangular lens having mirror surfaces (opticalsurfaces) 22 and 23 transferred from the transfer surfaces 13 a and 14a, respectively, of the mirror pieces or inserts 13 and 14,respectively. FIG. 3B shows the surface 24 of the lens 21 intended tosink (i.e., sink surface or B surface). A sinking area (or sunken area)24 a is indicated by cross-hatching. As shown, desired sinking occurs onthe sink surface or B surface 24 of the molded article or product 21(i.e., lens). As a result, the mirror surfaces 22 and 23 are desirablytransferred to the molded article or product 21 (i.e., lens), reducingthe internal strain of the molded article or product 21 (i.e., lens).

However, the injection mold assembly 10 has the following problems leftunsolved. If the various mold parts, namely, the stationary or uppermold 11, the movable or lower mold 12, the upper mirror insert 13, thelower mirror insert 14, the reference insert 15, and the sink insert 16,constituting the mold assembly 10, lack in accuracy, either individuallyor in combination, a gap d is formed between the parts, as shown in FIG.1. Then, air 9 is likely to flow into the cavity 17 via the gap d andprevent the desired surface from sinking. FIGS. 4A and 4B are viewssimilar to FIGS. 3A and 3B, showing a molded article or product 21(i.e., lens) molded with a stream of air 8 (see left side of FIG. 1)flowing into the side of the mold assembly 10 adjacent to the surface 15a of the reference insert 15 of the cavity 17 and against the C surface25 of the molded article or product 21. As shown, the molded article orproduct 21 (i.e., lens) fails to sink to a desired degree or practicallyfails to sink at its expected surface (i.e., sink surface or B surface24). In the worst case scenario as shown in FIGS. 4A and 4B, a C surface25 is caused to sink (see sunken area 25 a) and the surface 15 a of thereference insert 15 of the mold assembly 10 loses surface accuracy as atransfer surface.

Further, when air 9 is introduced into the cavity 17 via the vent hole18 by either natural draft or forced draft, it is apt to reach themirror surfaces 22 and 23 and cause them to sink also, depending on theamount of molten resin 20 filled in the cavity 17 or the amount of air9. A molded article or product 21 (i.e., lens) caused to sink as far asits mirror surfaces 22 and 23 is shown in a top plan view in FIG. 5A, ina side elevational view in FIG. 5B, and in a cross-sectional view, takenalong line 5 c—5 c of FIG. 5B, in FIG. 5C. As shown, a sunken region 24a formed in the sink surface or B surface 24 extends even to the mirrorsurface 22 and introduces a strain in the mirror surface 22, therebydeteriorating the ability of the molded article or product 21 (i.e.,lens).

Preferred embodiments of the present invention will be describedhereinafter.

FIG. 6 is a prior art figure showing the positional relationship betweenthe conventional molded article or product 21 and the vent hole 18 ofthe mold assembly 10. The relationship shown in FIG. 6 brings out theproblem discussed with reference to FIGS. 5A-5C. A first embodiment ofthe present invention eliminates such a problem by providing a moldingwith steps 26 between a vent hole 18 and transfer surfaces 13 a and 14 aof the mirror inserts 13 and 14, respectively.

First Embodiment

Specifically, FIG. 7A shows a specific molded article or product 21representative of the first embodiment. FIG. 7B is a partialcross-sectional view of an injection mold assembly 10 for producing themolded article or product 21. Basically, the mold assembly 10 is similarto the conventional mold assembly 10 shown in FIG. 1, and has itsstructural elements designated by the same reference numerals. Thedifference is that, as shown in FIG. 7B, the mold assembly 10 of thefirst embodiment includes a cavity 17 and indentations 6 formed in asink insert 16 between a vent hole 18 and transfer surfaces 13 a and 14a of the mirror inserts 13 and 14, respectively, wherein theindentations 6 are for forming steps 26 on the molded article or product21.

More specifically, the molded article or product 21 is implemented as arectangular lens having two mirror surfaces (optical surfaces) 22 and 23playing the role of lens surfaces. The molded article or product 21(i.e., lens) includes a surface 24 that will be allowed to sink (i.e.,sink surface or B surface). The steps 26 are formed on the surface 24between 22 and 23 from the vent hole 18.

As shown in FIG. 7B, the sink insert 16 of the mold assembly 10 includesthe indentations 6 complementary in configuration to the steps 26 of themolded article or product 21 (i.e., lens) and formed on the moldsurface, respectively. Just after the molten resin or molten moldingmaterial 20 has been injected into the cavity 17 (between mirror inserts13 and 14, reference insert 15, and sink insert 16), air 9 is forced outof the cavity 17 via the vent hole 18 and a bore 19 due to the highinternal pressure of the molten resin 20. The internal pressure of themolten resin 20 sequentially decreases as the molten resin 20 is cooled.When the pressure of the molten resin 20 decreases below the atmosphericpressure or below a compression pressure (i.e., when a compressorcommunicates with the vent hole 18 via the bore 19), air 9 begins toflow into the cavity 17 via the vent hole 18, causing the molten resin20 to sink away from the vent hole 18. If the molten resin 20 did notinclude the steps 26, the molten resin 20 might sink as far as thetransfer surfaces 13 a and 14 a of the mirror inserts 13 and 14,respectively. In the illustrative embodiment, the steps 26, whichisolate the mirror surfaces 22 and 23 from the vent hole 18, tend tocontract toward each other, as indicated by the arrows in FIG. 7B.However, the indentations 6 of the sink insert 16 of the mold assembly10 interfere with the steps 26 and prevent the steps 26 fromcontracting. As a result, the molten resin 20 and mold assembly 10remain in close contact with each other and prevent sinking fromproceeding over the steps 26 (i.e., confine the sinking to the regionbetween the steps 26). The molded article or product 21 (i.e., lens) istherefore caused to sink only 22 and 23 by means of a transfermechanism.

Second Embodiment

FIG. 8 shows a molded article or product 21 representative of a secondembodiment of the present invention and produce by injection molding.The molded article or product 21 is also implemented as a lens similarin configuration to the lens of FIG. 7A. As shown, a step 26 is formedon the sink surface or B surface 24 of the molded article or product 21(i.e., lens) adjoining the vent hole (not shown) such that the step 26surrounds the vent hole (not shown). The step 26 surrounding the venthole (not shown) prevents air 9 from turning around and allows thesinking region to be controlled more positively than in the firstembodiment.

Third Embodiment

FIGS. 9A-9C show a molded article or product 21 representative of athird embodiment of the present invention. The molded article or product21 is also implemented as a lens similar in configuration to the lens ofFIG. 7A. As shown, a step 26 is formed on the sink surface or B surface24 of the molded article or product 21 adjoining the vent hole (notshown). The top portion of the step 26 has a substantially similarcontour as the contour of the upper portion of the sink surface or Bsurface 24. Specifically, when the molded article or product 21 is arectangular lens, the step 26 surrounds the vent hole (not shown) andhas an upper portion with a contour similar to the contour of the upperportion of the sink surface or B surface 24 of the molded article orproduct 21 (i.e., lens). The degree of sinking of a sunken region 24 acan therefore be controlled in a ratio similar to the cross-sectionalarea of each cross-section of the molded article or product 21 (seesection A1 or A2 as shown in FIGS. 9A, 9B, and 9 c). This successfullymakes the internal strain and surface accuracy uniform and therebyenhances the accuracy of the molded article or product 21 (i.e., lens).

As shown in FIG. 10A, the steps 26, of the first to third embodiments,are each represented by projections extending outwardly from the sinksurface or B surface 24 of the molded article or product 21 (i.e.,lens). Alternatively, as shown in FIG. 10B, the steps 26 may berepresented by recesses formed in the extending into the sink surface orB surface 24 of the molded article or product 21 (i.e., lens). The steps26 implemented as recesses are for controlling the sinking region 24 a.When the steps 26 are recesses, as shown in FIG. 10B, the mold assembly10 will be formed with outwardly extending projections around the venthole 18 (instead of inwardly extending projections as shown in FIG. 7B).

Fourth Embodiment

FIGS. 11A and 11B show a molded article or product 21 representative ofa fourth embodiment of the present invention. As shown, steps 26 areformed on the mirror surfaces 22 and 23 of the molded article or product21. This configuration prevents air 9 from reaching the mirror surfaces22 and 23 via the vent hole (not shown) because the steps 26 of themolded article or product 21 and the indentations 6 of the mold assembly(not shown) remain in close contact with each other.

Fifth Embodiment

FIGS. 12A and 12B show a molded article or product 21 representative ofa fifth embodiment of the present invention. As shown, a step 26 isformed on a part of a mirror surface 22 or 23. Specifically, when it isknown that air 9 will turn around to a part of the mirror surface 22 or23 molded article or product 21 and cause it to sink beforehand, thestep 26 may be formed only in such a part of the mirror surface 22 or23. This configuration saves cost when a mold assembly (not shown) isformed with an indentation 6.

Sixth Embodiment

FIG. 13A shows a molded article or product 21 representative of a sixthembodiment of the present invention. FIG. 13B is a cross-sectional viewtaken along plane 13 b—13 b—13 b of FIG. 13A. As shown, steps 26 areformed on opposite edges of the mirror surface 22 contiguous with thesink surface or B surface 24 and C surface 25. Likewise, steps 26 areformed on the opposite edges of the other mirror surface 23 contiguouswith the sink surface or B surface 24 and C surface 25. As shown in FIG.1, when the mold assembly 10 is not accurate, air is apt to enter thecavity 17 via an unexpected portion. As a result, as shown in FIGS. 14Aand 14B specifically, air is likely to flow into the side of the cavity17 of the mold assembly 10 adjacent to the reference surface 15 a of thereference insert 15 (i.e., C surface of the molded article or product21) so that the air turns around to the mirror surfaces 22 and 23 tocause them to sink. In the illustrative embodiment, the steps 26 formedon both longitudinal edges of the mirror surface 22 and those of themirror surface 23 prevent air from turning around to the mirror surfaces22 and 23 and thereby prevents the mirror surfaces 22 and 23 fromsinking.

Seventh Embodiment

FIG. 14A shows a molded article or product 21 representative of aseventh embodiment of the present invention. FIG. 14B is across-sectional view taken along plane 14 b—14 b—14 b of FIG. 14A. Asshown, the molded article or product 21 is identical with the moldedarticle or product 21 shown in FIG. 13A, except that the steps 26,facing each other of the mirror surfaces 22 and 23, are replaced with asingle step 26 surrounding the mirror surface 22 or 23. The steps 26 canobstruct air 9 more positively and can therefore prevent the mirrorsurfaces 22 and 23 from sinking more positively.

Eighth Embodiment

To obstruct air 9 tending to reach the mirror surfaces 22 and 23 of themolded article or product 21, the steps 26, shown in FIG. 13B or 14B,suffice. FIG. 15 shows a molded article or product 21 configured to beeasily separable from a mold assembly 10. As shown, the steps 26 areeach of a height h extending from the mirror surfaces 22 and 23. Withthe steps 26, the molded article or product 21 can be easily separatedfrom a mold assembly 10, while preserving its accuracy.

Ninth Embodiment

FIGS. 16A and 16B each shows a particular molded article or product 21representative of a ninth embodiment of the present invention. As shown,steps 26, facing each other at both edges of each mirror surface 22 or23, are provided with a triangular cross-section (FIG. 16A) or anaccurate cross-section (FIG. 16B). This not only enhances the partingability of the molded article or product 21, but also simplifies theprocedure for forming indentations 6 in, e.g., the mirror pieces 13 and14 of the mold assembly 10.

In each of the eighth and ninth embodiments, the steps 26 each have aheight h greater than 0.1 mm inclusive. Experiments showed that heightsh greater than 0.1 mm inclusive can sufficiently obstruct air.

Tenth Embodiment

This embodiment relates to a method of forming a molded article orproduct 21 and will be described with reference to FIG. 7B. First, themovable or lower mold 21, carrying the mirror piece or insert 14 and thereference insert 15 therewith, is brought into contact with thestationary or upper mold 11, loaded with the other mirror insert 13 andthe reference insert 15. As a result, the mold surfaces of the molds 11and 12 form the cavity 17 having a preselected volume. A gate or sprue(not shown) is formed in a mold surface (not shown) of the mold assembly10 in order to inject the molten resin 20 into the cavity 17. Aconventional filling machine (not shown) is connected to the gate orsprue in order to fill the cavity 17 with the molten resin 20 byinjection.

Just after molten resin or molten molding material 20 has been injectedinto the cavity 17, air is forced out of the cavity 17 via the vent hole18 and a bore 19 due to the high internal pressure of the molten resin20. The internal pressure of the molten resin 20 decreases below theatmospheric pressure or below a compression pressure (i.e., when acompressor communicates with a vent hole 18 via a bore 19), air 9 beginsto flow into the cavity 17 via the vent hole 18, causing the moltenresin 20 to sink away from the vent hole 18. At this instant, the steps26, isolating the mirror surfaces 22 and 23 from the vent hole 18, asshown in, e.g., FIG. 7A, tend to contract toward each other, asindicated by arrows in FIG. 7B. However, the indentations 6 of the moldassembly 10 interfere with the steps 26 and prevent them fromcontracting. As a result, the molten resin 20 and mold assembly 10remain in close contact with each other and prevent sinking fromproceeding over the steps 26.

FIG. 17 shows how the internal pressure of the molten resin 20 variesfrom the time when the molten resin 20 begins to be injected into thecavity 17 to the time when it is fully cooled off. In the case where air9 under pressure is fed via the vent hole 18, its pressure is switchedin a manner also shown in FIG. 17. As shown, in the illustrativeembodiment, air 9 is continuously fed even after the internal pressureof the molten resin 20 has been lowered to zero, generating air pressurein the vent hole portion. Experiments showed that the air pressurecontinuously generated, even after the drop of the resin pressure tozero, allows the sinking region 24 a to be surely controlled.

More specifically, the molten resin 20 remains in close contact with themold assembly 10 until the internal pressure of the molten resin 20drops to zero, and sinking occurs thereafter. It is therefore necessaryto apply air that has been pressured for some extra period of time afterthe internal pressure has dropped to zero. It was found that when themolded article or product 21 is a lens, as shown and described, thesinking region 24 a can be controlled if the air pressure iscontinuously applied for at least 5 seconds more after the drop of theinternal pressure of the molten resin 20 to zero. The air pressureshould preferably be higher than the atmospheric pressure (about 0.1MPa) inclusive, but lower than 2 MPa inclusive.

The first to tenth embodiments, shown in the figures and describedabove, achieve the following various unprecedented advantages.

(1) In a molded article or product formed by an injection mold assembly,in which a pressure difference or an air pressure is generated betweenmirror surface portions corresponding to the mirror surfaces of a moltenmolding material or molten resin and a vent hole portion correspondingto a vent hole in order to cause the molten molding material or moltenresin to sink, a step is formed in a cavity between the vent hole and amirror surface portion. When the molten molding material or molten resinis cooled, the step prevents the resin from contracting over the stepand thereby guarantees adhesion of the portions of the molding materialother than a surface expected to sink and the mold assembly. Thisprevents sinking from proceeding over the step and thereby confines itto a region delimited by the step.

(2) The step is provided on the surface of the molded article or productfacing the vent hole, so that the sinking region can be confined to sucha surface.

(3) Two steps are formed in such a manner as to isolate the vent holeand the mirror surface portions, so that sinking is prevented fromextending to the mirror surfaces.

(4) The step is formed to surround the vent hole in order to prevent airfrom turning around to the surface portions. This allows sinking to beconfined to the region delimited by the step and thereby preventssinking from extending to the mirror surfaces.

(5) When the step has a portion with a contour similar to the contour ofthe side of the molded article or product facing of the vent hole, thesinking region can be controlled in the same ratio as thecross-sectional area of the molded article or product. This makes theinternal strain and surface accuracy of the molded article or productuniform and therefore enhances the accuracy of the molded article orproduct.

(6) The sinking region can be controlled both when the step or steps ofthe molded article or product are formed as projections and when theyare formed as recesses.

(7) The step or steps prevent air from reaching the mirror surfaceportions via the vent hole and thereby protects the mirror surfaceportions from sinking.

(8) The steps configured to face each other at opposite longitudinaledges of each mirror surface obstruct air coming through the vent holeor any other portion of the mold assembly. This also surely protects themirror portions from sinking.

(9) The steps are each configured to surround the associated mirrorsurface portion. This prevents air from reaching the mirror surfacesmore positively and prevents the sinking of the mirror surface portionsmore positively.

(10) The steps are tapered in order to confine the sinking to the areadelimited by the steps. In addition, the tapering of the steps enhancesthe parting ability of the molded article or product from the moldassembly.

(11) The steps are provided with a triangular or an arcuatecross-section in order to enhance the parting ability of the moldedarticle or product from the mold assembly and to facilitate theformation of steps in the mold assembly.

(12) In a method of forming a molded article or product of the kinddescribed, an air pressure is continuously generated via the vent holeeven after the internal pressure of the molten resin in the cavity hasdropped to zero so as to control the sinking region more positively.

Eleventh Embodiment

FIG. 18 shows an injection mold assembly 10 representative of aneleventh embodiment of the present invention. As shown, a sink insert 16is located at a position where sinking is expected to occur. The sinkinsert 16 is formed with a vent hole 18, a bore 19 communicating withthe vent hole 18, and a pair of exhaust holes 60 positioned above andbelow the vent hole 18 and the bore 19, respectively. FIG. 19 shows apositional relationship between the vent hole 18 and the exhaust holes60.

In the illustrative embodiment, just after molten resin or moltenmolding material 20 has been injected into a cavity 17, it is difficultfor air 9, fed under pressure via the vent hole 18, to enter the cavity17. The internal pressure of the resin 20 sequentially decreases as theresin 20 is cooled. When the pressure of the molten resin 20 decreasesbelow the pressure of the compressed air 9 delivered to the vent hole18, the air 9 begins to flow into the cavity 17 via the vent hole 18. Asa result, the portion of the molten resin 20 corresponding to one side(i.e., sink surface or B surface 24) of a molded article or product 21,as shown in FIG. 19, and facing the vent hole 18, begins to sink (seeFIG. 18, X4 shows the distance of sinking) away from the inner peripheryof the cavity 17. The compressed air 9, introduced into the cavity 17,hits against the molten resin 20 and then, is discharged from the cavity17 via the exhaust holes 60. That is, the compressed air 9 is preventedfrom turning around to the upper mirror surface 22 and lower mirrorsurface 23 of the molded article or product 21. If desired, a machinefor forced exhaustion may be connected to the bore 19 in order topromote discharge of the compressed air 9 more effectively. When themold assembly 10 is used to form, e.g., a lens of resin applicable to animage forming apparatus or similar optical apparatus, the exhaust holes60 should be 0.001 mm to 0.5 mm wide (vertical dimension in FIG. 18).With such a width, the exhaust holes 60 allow a minimum of resin toenter therein and thereby frees the molded article or product 21 fromburrs.

Twelfth Embodiment

FIG. 20 shows a mold assembly 10 representative of a twelfth embodimentof the present invention. As shown, this embodiment is identical withthe eleventh embodiment, except that the exhaust holes 60 are formed byporous member 61.

Thirteenth Embodiment

FIG. 21 shows the position of an exhaust hole 61 formed in an injectionmold assembly 10 representative of a thirteenth embodiment of thepresent invention. As shown, a continuous exhaust hole 60 is formed tosurround the vent hole 18. The exhaust hole 60 may also be formed by theporous member 61 in order to simplify the configuration of the sinkinsert 16, as shown in FIG. 20.

Fourteenth Embodiment

FIG. 22 shows a fourteenth embodiment of the present invention which isa modification of the thirteenth embodiment. As shown, this embodimentis identical with the thirteenth embodiment, except that an upperportion of an exhaust hole 60 has a similar contour to the contour of anupper portion of the side of the molded article or product 21 which isexpected to sink (i.e., sink surface or B surface 24). Again, theexhaust hole 60 may be formed by the porous member 61 in order tosimplify the configuration of the sink insert 16, as shown in FIG. 20.

The eleventh to the fourteenth embodiments, shown in the figures anddescribed above, have the following unprecedented advantages.

(1) At least one exhaust hole is formed in the vicinity of a vent holeused to feed air under pressure for causing sinking. The exhaust holedischarges air, which causes sinking to occur in the vicinity of thevent hole, to the outside of a mold assembly before it reaches portionsexpected to form mirror surfaces. Therefore, air is prevented fromreaching portions other than the portion expected to sink, so that theshape of the mold assembly is surely transferred to the other portionsof the molding.

(2) A single exhaust hole surrounds the vent hole and discharges air,which causes sinking to occur smoothly, to the outside of the moldassembly. This guides air only to the portion of the molding expected tosink more positively.

(3) The exhaust hole is or the exhaust holes are formed by a porousmember. Therefore, particularly when a single exhaust hole surrounds thevent hole, the porous member, formed with the holes, is simple instructure.

(4) Air is forcibly discharged via the exhaust holes so that air, whichcauses sinking to occur in the cavity, can be discharged more smoothly.

(5) The exhaust hole has an opening width as small as 0.001 mm to 0.5 mmand prevents a molding material from entering it. This frees theresulting molding from burrs.

Fifteenth Embodiment

FIGS. 23A-23C show a plastic molded article or product 21 formed by amethod representative of a fifteenth embodiment of the presentinvention. The molded article or product 21 may be implemented, not onlyas a lens, but also as a mirror, prism or similar optical device. Asshown, the molding molded article or product 21 has mirror surfaces 22and 23 on its top and bottom, respectively. In addition, the moldingmolded article or product 21 has a reference surface or non-transfersurface or C surface 25 at one side and a sink surface or non-transfersurface or B surface 24 at the other side. The reference surface 25 isto be mounted to another part, while the sink surface 24 is expected tosink.

FIGS. 23A-23 C show a plastic molded article or product 21 formed by amethod representative of a fifteenth embodiment of the presentinvention. The molded article or product 21 may be implemented, not onlyas a lens, but also as a mirror, prism or similar optical device. Asshown, the molding molded article or product 21 has mirror surfaces 22and 23 on its top and bottom, respectively. In addition, the moldingmolded article or product 21 has a reference surface or non-transfersurface of C surface 25 at one side and a sink surface or non-transfersurface or B surface 24 at the other side. The reference surface 25 isto be mounted to another part, while the sink surface 24 is expected tosink.

Reference will be made to FIGS. 24A, 24B, 25A, and 25B for describing amold assembly 10 for producing the above molded article or product 21.As shown, the mold assembly 10 includes a stage 3 loaded with a movableor lower mold 12. A stationary or upper mold 11 is positioned above themovable or lower mold 12. The movable or lower mold 12 is movable intoand out of contact with the stationary or upper mold 11 by being drivenby a clamping device (not shown).

A plurality (four in the embodiment) of inserts, namely, mirror piecesor inserts 13 and 14, reference insert 15, and sink insert 16, areinterposed between the movable or lower mold 12 and the stationary orupper mold 11 and constitute cavity inserts. Specifically, upper mirrorpiece or insert 13 and lower mirror piece or insert 14 face each otherand are formed with transfer surfaces 13 a and 14 a, respectively, forforming the mirror surfaces 22 and 23, respectively, of the moldedarticle or product 21. A reference insert 15 and a sink insert 16 faceeach other at both sides of the upper mirror piece or insert 13 and thelower mirror piece or insert 14 and are formed with non-transfersurfaces 15 a and 16 a, respectively, in order to form the referencesurface or C surface 25 and sink surface or B surface 24, respectively.The surfaces 13 a, 14 a, 15 a, and 16 a of the upper mirror piece orinsert 13, lower mirror piece or insert 14, reference insert 15, andsink insert 16 together form a cavity 17. The non-transfer surfaces 15 aand 16 a are each formed with fine irregularities or undulations 62.

It is noted that FIGS. 24A and 24B show only one half of the moldassembly 10. The other half is also provided with cavity insertsidentical with the upper mirror piece or insert 13, lower mirror pieceor insert 14, reference insert 15, and sink insert 16. A gate or sprue(not shown) is formed in the stationary or upper mold 11, while a gateor sprue 63 is formed in the upper mirror piece or insert 13 and iscommunicable to the gate or sprue (not shown) of the stationary or uppermold 11. An injection molding machine (not shown) feeds molten resin 20to the cavity 17 via the gate or sprue (not shown) of the stationary orupper mold 11 and the gate or sprue 63.

A vent hole 18 is formed in the sink insert 16. The vent hole 18 is opento the cavity 17 at one end and connected to a feed tube 64 at the otherend. The feed tube 64 is interposed between the movable or lower mold 12and the stationary or upper mold 11 and connected to a gas feed unit 65via a temperature control unit 66. A gas (e.g., air 9 compressed to apreselected pressure by the gas feed unit 65 and controlled to apreselected temperature by the temperature control unit 66) is fed viathe feed tube 64.

In the illustrative embodiment, molten resin 20, heated above itssoftening point, is injected into the cavity 17 of the mold assembly 10heated to a temperature lower than the softening point of the resin.Therefore, the temperature control unit 66 controls the gas to atemperature about 3° C. lower than the temperature of the mirror piecesor inserts 13 and 14 and reference insert 15. It follows that thetemperature of the gas, fed from the feed tube 64 to the sink surface16, is lower than the temperature of the mirror surfaces 22 and 23 andreference surface 25. The movable or lower mold 12 and the stationary orupper mold 11, surrounding the mirror inserts 13 and 14, the referenceinsert 15, and the sink insert 16, are each provided with a temperaturecontrol mechanism, including a heater and an oil cooler (not shown). Theheater and oil cooler heat and cool, respectively, the associated molds11 and 12, respectively, and therefore, the mirror inserts 13 and 14,the reference insert 15, and the sink insert 16.

In the illustrative embodiment, the temperature control unit 66 and gasfeed unit 65 constitute a feeding device, and also constitute gasfeeding means in combination with the vent hole 18.

The operation of the above-described arrangement will be discussedhereinafter. When a lens or similar plastic optical element is producedby conventional injection molding, molding conditions allowing theentire area to be transferred (i.e., allowing the internal pressure ofthe molding to drop substantially to zero at the time of take-out) areset up. However, because molten resin is sharply cooled as soon as it isintroduced into a mold, the resulting temperature distribution, pressuredistribution, density distribution and so forth disturb the shape of themolding. This, coupled with the internal strain (deflection) of theresin, adversely influences the optical characteristic of the molding.Although during the transfer of the mold configuration, internal strainand deformation may be reduced if a molding is caused to partly sink, itis extremely difficult to specify the part of a molding to sink. Thisembodiment is significant in that it can specify the part of a moldingto sink, as follows.

While the mold assembly 10 is held at a temperature lower than thesoftening point of resin, molten resin 20, heated above its softeningpoint, is injected into the cavity 17. Then, a resin pressure is causedto act on the transfer surfaces 13 a and 14 a of the mirror inserts 13and 14, respectively. At the same time as the injection of the moltenresin 20, a cool gas, compressed to a preselected pressure by the airfeed unit 65 and controlled to a preselected temperature by thetemperature control unit 66, is fed to the sink surface or B surface 24via the vent hole 18. The feed of the gas, such as air, is continueduntil the molten resin 20 has been fully injected into the cavity 17. Atthis instant, the sink surface or B surface 24, lower in temperaturethan the mirror surfaces 22 and 23, solidifies first and increases itsviscosity. This makes it difficult for the sink surface or B surface 24to remain in contact with the non-transfer surface 16 a of the sinkinsert 16 before the end of the injection of the molten resin 20. Afterthe injection of the molten resin 20 and the subsequent stop of thefeeding of the cool gas, such as air, the cavity 17 is caused to dwellat a preselected pressure and cooled. As soon as the pressure inside thecavity 17 drops substantially to zero, the stationary or upper mold 11is pulled away or separated from the movable or lower mold 12.Subsequently, the resulting molded article or product 21 is taken out ofthe cavity 17.

The sink surface or B surface 24 of the molded article or product 21obtains a parting ability earlier than the other surfaces of the moldedarticle or product 21. As a result, the sink surface or B surface 24beings to sink earlier than the other surfaces (i.e., the mirrorsurfaces 22 and 23, and the reference or C surface 25) contacting theupper and lower mirror pieces or inserts 13 and 14, the reference insert15, and the sink insert 16. This successfully prevents the mirrorsurfaces 22 and 23 from sinking and thereby allows the desired mirrorsurfaces 22 and 23 to be faithfully transferred to the molded article orproduct 21 in a short molding cycle.

Moreover, the sink surface or B surface 24 is held at a temperaturelower than the temperature of the molten resin 20 from the end of theresin injection to the beginning of cooling. Consequently, a temperaturedifference does not occur between the mirror surfaces 22 and 23 and thesink surface or B surface 24 during cooling, so that an internal strainis prevented from remaining in the molded article or product 21 afterthe opening of the mold assembly 10. This is not only prevents theaccuracy of the mirror surfaces 22 and 23 from decreasing, but alsoprevents the entire molded article or product 21 from deforming.

In addition, the gas feeding means can be implemented only if the venthole 18 is formed in the sink insert 16 and connected to both thetemperature control unit 66 and gas feed unit 65. The mold assembly 10is therefore simple in construction.

Sixteenth Embodiment

Referring to FIGS. 26, 27A-27D, and 28A-28D, a method and an apparatusfor producing a plastic molded article or product 21 representative of asixteenth embodiment of the present invention will be described. Themolded article or product 21 to be provided by this embodiment isidentical in configuration with the molded article or product 21 of thefifteenth embodiment and will be described with reference to FIGS.23A-23C. Structural elements identical with the elements of thefifteenth embodiment are designated by identical reference numerals andwill not be described specifically in order to avoid redundancy.

As shown in FIGS. 26 and 27A-27D, a plurality of (four in theembodiment) inserts are interposed between the movable or lower mold 12and the stationary or upper mold 11 and constitute cavity inserts.Specifically, mirror pieces or inserts 13 and 14 face each other and areformed with transfer surfaces 13 a and 14 a, respectively, for formingthe mirror surfaces 22 and 23, respectively, of the molded article orproduct 21. A reference insert 15 and a sink insert 16 face each otherat both sides of the mirror pieces or inserts 13 and 14 and are formedwith non-transfer surfaces 15 a and 16 a, respectively, in order to formthe reference surface or C surface 25 and sink surface or B surface 24,respectively, of the molded article or product 21. The transfer andnon-transfer surfaces 13 a, 14 a, 15 a, and 16 a of the mirror pieces orinserts 13 and 14, the reference insert 15, and the sink insert 16 forma cavity 17. The non-transfer surfaces 15 a and 16 a of the referenceinsert 15 and sink insert 16, respectively, are each formed with fineirregularities or undulations 62.

A gate or sprue (not shown) is formed in the stationary or upper mold11, while a gate or sprue 63 is formed in the mirror insert 13 and iscapable of communication with the gate or sprue (not shown) in thestationary or upper mold gate or sprue (not shown) of the stationary orupper mold 11 and the gate or sprue 63. A vent hole 18 is formed in thesink insert 16. The vent hole 18 is open to the cavity 17 at one end andconnected to a feed tube 64 at the other end. The feed tube 64 isinterposed between the movable or lower mold 12 and the stationary orupper mold 11.

The feed tube 64 is connected to a gas feed unit 65. The gas feed unit65 feeds a gas, e.g., air 9 compressed to a preselected pressure, tobetween the sink surface 24 and the transfer surface 16 a of the sinkinsert 16, via the feed tube 64 and vent hole 18. In this embodiment,the gas feed unit 65 constitutes a feeding device and constitutes gasfeeding means in combination with the vent hole 18 and feed tube 64.

The operation of the illustrative embodiment will be described withreference to FIGS. 27A-27D. As shown, while the mold assembly 10 is heldat a temperature lower than the softening point of resin, molten resin20, heated above its softening point, is injected into the cavity 17.Then, a resin pressure is caused to act on the transfer surfaces 13 aand 14 a of the mirror inserts 13 and 14, respectively. At the same timeas the injection of the molten resin 20, a gas, i.e., air 9 compressedto a preselected pressure by the air feed unit 65, is fed to between thesink surface 16 a of the sink insert 16 and the non-transfer surface 15a of the reference insert 15. The feed of the gas, e.g., air 9, iscontinued until the molten resin 20 has been fully injected into thecavity 17 (see FIGS. 27A and 27B). At this instant, a gas layer isformed between the non-transfer surface 16 a of the sink insert 16 andthe sink surface 24 of the molded article or product 21, making ifdifficult for the sink surface 24 to remain in contact with thenon-transfer surface 16 a of the sink insert 16 before the end of theinjection of the molten resin 20.

After the injection of the molten resin 20 and the following stop offeed of the gas, e.g. air 9, the cavity 17 is caused to dwell at apreselected pressure and cooled. As a result, the gas layer between thesink surface or B surface 24 of the molded article or product 21 and thenon-transfer surface 16 a of the sink insert 16 is compressed by theinternal pressure of the molten resin 20, but remains between them (seeFIG. 27C). Such residual gas expands as the internal pressure approacheszero, separating the sink surface or B surface 24 of the molded articleor product 21 from the non-transfer surface 16 a of the sink insert 16.When the internal pressure reaches zero, the non-transfer surface 16 aof the sink insert 16 obtains a parting ability earlier than the othersurfaces (i.e., transfer surfaces 13 a and 14 a of mirror pieces orinserts 13 and 14, respectively, and reference surface 15 a of referenceinsert 15). When the pressure inside the cavity 17 drops substantiallyto zero, the stationary or upper mold 11 is released from the movable orlower mold 12. Subsequently, the molded article or product 21 is takenout of the cavity 17.

In this manner, the sink surface or B surface 24 of the molded articleor product 21 begins to sink earlier than the other surfaces (i.e.,transfer surfaces 13 a, 14 a, and reference surface 15 a) contacting themirror pieces or inserts 13 and 14 and the reference insert 15,respectively. This successfully prevents the mirror surfaces 22 and 23of the molded article or product 21 from sinking and thereby allows thedesired mirror surfaces 22 and 23 to be faithfully transferred to themolded article or product 21 in a short molding cycle. Moreover, the gaslayer remains between the sink surface or B surface 24 of the moldedarticle or product 21 and the non-transfer surface 16 a of the sinkinsert 16 until the cooling step begins after the injection of themolten resin 20, preventing the pressure difference, between the mirrorsurfaces 22 and 23 of the molded article or product 21 and the sinksurface or B surface 24 of the molded article or product 21, fromincreasing during cooling. Consequently, the internal strain of themolded article or product 21 is prevent from remaining, after theopening of the mold assembly 10. This not only prevents the accuracy ofthe mirror surfaces 22 and 23 of the molded article or product 21 fromdecreasing, but also prevents the entire molded article or product 21from deforming.

This embodiment may be practiced with the same configuration as thefifteenth embodiment, as follows. The gas fed from the gas feed unit 65is controlled to substantially the same temperature as the mold assembly10 by the temperature control unit 66 shown in FIG. 15. In this case, asshown in FIGS. 28A-28D, while the mold assembly 10 is held at atemperature lower than the softening point of resin, molten resin 20,heated above its softening point, is injected into the cavity 17. Then,a resin pressure is caused to act on the transfer surfaces 13 a and 14 aof the mirror pieces or inserts 13 and 14, respectively. At the sametime as the injection of the molten resin 20, the gas, i.e., air 9compressed to a preselected pressure by the gas feed unit 65, is fed tobetween the sink surface 24 of the molded article or product 21 and thenon-transfer surface 16 a of the sink insert 16. The feed of the gas,e.g., air 9, is continued until the molten resin 20 has been fullyinjected into the cavity 17 (see FIGS. 28A and 28B). At this instant, agas layer is formed between the non-transfer surface 16 a of the sinkinsert 16 and the sink surface 24 of the molded article or product 21,making it difficult for the sink surface 24 of the molded article orproduct 21 to remain in contact with the non-transfer surface 16 a ofthe sink insert 16 before the end of the injection of the molten resin20.

After the injection of the molten resin 20 and the subsequent stop offeed of the gas, the cavity 17 is caused to dwell at a preselectedpressure and cooled. As a result, the gas layer, between the sinksurface 24 of the molded article or product 21 and the non-transfersurface 16 a of the sink insert 16, is compressed by the internalpressure of the molten resin 20, but remains between them (see FIG.28C). The residual gas expands as the internal pressure approaches zero,separating the sink surface 24 of the molded article or product 21 fromthe non-transfer surface 16 a of the sink insert 16. When the internalpressure reaches zero, the non-transfer surface 16 a of the sink insert16 obtains a parting ability earlier than the other surfaces, i.e.,mirror surfaces 22 and 23 and reference or C surface 25 of the moldedarticle or product 21 (see FIG. 28D). When the pressure inside thecavity 17 drops substantially to zero, the stationary or upper mold 11is released from the movable or lower mold 12. Subsequently, the moldedarticle or product 21 is taken out of the cavity 17.

Seventeenth Embodiment

A method and an apparatus for producing a plastic molded article orproduct 21 representative of a seventeenth embodiment of the presentinvention will be described with reference to FIGS. 29, 30A, and 30B.The molded article or product 21 to be produced by this embodiment isidentical in configuration with the molded article or product 21 of thefifteenth embodiment and will be described in reference to FIGS.23A-23C. Structural elements identical with the elements of thefifteenth embodiment will be designated by identical reference numeralsand will not be described specifically in order to avoid redundancy.

As shown in FIGS. 29, 30A, and 30B, a plurality (four in the embodiment)of inserts namely, mirror pieces or inserts 13 and 14, reference insert15, and sink insert 16, are interposed between the movable or lower mold12 and the stationary or upper mold 11 to form a cavity 17.Specifically, mirror pieces or inserts 13 and 14, which face each otheracross the cavity 17, are formed with transfer surfaces 13 a and 14 a,respectively, for forming the mirror surfaces 22 and 23, respectively,of the molded article or product 21 (see FIGS. 23A-23C). The referenceinsert 15 has a reference surface 15 a for forming a reference or Csurface 25 on the molded article or product 21 and the sink insert 16has a non-transfer surface 16 a for forming a sink or B surface 24 onthe molded article or product 21. The surfaces 13 a, 14 a, 15 a, and 16a of the inserts 13, 14, 15, and 16, respectively, form the cavity 17.The reference surface 15 a and the non-transfer 16 a of the referenceinsert 15 and the sink insert 16, respectively, are each formed withfine irregularities or undulations 62.

A gate or sprue (not shown) is formed in the stationary or upper mold11, while a gate or sprue 63 is formed in the lower mirror piece orinsert 16 of the movable or lower mold 12 and is capable ofcommunication with the gate or sprue (not shown) of the stationary orupper mold 11. An injection molding machine (not shown) injects moltenresin 20 into the cavity 17 via the gate or sprue (not shown) of thestationary or upper mold 11 and the gate or sprue 63 through the lowermirror piece or insert 14 formed in the movable or lower mold 12. A venthole 18 is formed in the sink insert 16. One end of the vent hole 18communicates with a gas feed unit 65 via a vent hole 18 formed in themovable or lower mold 12 and a feed tube 64. The other end of the venthole 18 communicates with the outside of the mold assembly 10 via anexhaust tube 60.

The gas feed unit 65 feeds gas, e.g., air 9 controlled to a preselectedpressure and a preselected temperature, to the vent hole 18, via thefeed tube 64 and vent hole 18, and then discharges it via the vent hole18 and exhaust tube 60. The gas, e.g., air 9 controlled to a preselectedpressure and a preselected temperature, therefore cools the non-transfersurface 16 a of the sink insert 16 of the mold assembly 10.

In the illustrative embodiment, molten resin 20, heated above itssoftening point, is injected into the cavity 17 of the mold assembly 10heated to a temperature lower than the softening point of the resin.Therefore, a temperature control unit 66 controls the temperature of thegas to a temperature about 3° C. lower than the temperature of themirror pieces or inserts 13 and 14 and reference insert 15. It followsthat the temperature of the gas, fed from the vent hole 18 to the sinksurface or B surface 24 of the molded article or product 21, is lowerthan the temperature of the transfer surface 13 a and 14 a of the mirrorpieces or inserts 13 and 14, respectively, and reference surface 15 a ofthe reference insert 15.

In this embodiment, the gas feed unit 65, feed tube 64, vent hole 18,and exhaust tube 60 constitute cooling means.

In operation, before the injection of molten resin 20, a cool gas, e.g.,air 9 controlled to a preselected pressure and a preselectedtemperature, is fed from the gas feed unit 65 to a non-transfer surface16 a via the vent hole 18 so as to cool the non-transfer surface 16 a.Then, while the mold assembly 10 is held at a temperature lower than thesoftening point of resin, molten resin 20, heated above its softeningpoint, is injected into the cavity 17. Subsequently, a resin pressure iscaused to act on the transfer surfaces 13 a and 14 a of the mirrorpieces or inserts 13 and 14, respectively. The feed of the cool gas,e.g., air 9, is continued until the molten resin 20 has been fullyinjected into the cavity 17. At this instant, the sink surface or Bsurface 24 of the molded article or product 21, lower in temperaturethan the mirror surfaces 22 and 23 of the molded article or product 21,solidifies first and increases its viscosity, making it difficult forthe sink surface or B surface 24 to remain in contact with thenon-transfer surface 16 a of the sink insert 16 before the end of theinjection of the molten resin 20. After the injection of the moltenresin 20 and the subsequent stop the of feeding of the cool gas, e.g.,air 9, the cavity 17 is caused to dwell at a preselected pressure andcooled. When the pressure inside the cavity 17 drops substantially tozero, the stationary or upper mold 11 is released from the movable orlower mold 12. Subsequently, the molded article or product 21 is takenout of the cavity 17. This embodiment achieves the same advantages asthe fifteenth embodiment.

Eighteenth Embodiment

A method and an apparatus for producing a plastic molded article orproduct 21 representative of a seventeenth embodiment of the presentinvention will be described with reference to FIGS. 31, 32A-32C, 33A,33B, 34A-34C, and 35. The molded article or product 21 to be produced bythis embodiment is identical in configuration with the molded article orproduct 21 of the fifteenth embodiment and will be described withreference to FIGS. 23A-23C. Structural elements identical with theelements of the fifteenth embodiment will be designated by identicalreference numerals and will not be described specifically in order toavoid redundancy.

As shown, a plurality (four in the embodiment) of inserts, namely,mirror pieces or inserts 13 and 14, reference insert 15, and sink insert16, are interposed between the movable or lower mold 12 and thestationary or upper mold 11 and form a cavity 17. Specifically, mirrorpieces or inserts 13 and 14, which face each other across the cavity 17,are formed with mirror surfaces 13 a and 14 a, respectively, for formingthe mirror surfaces 22 and 23, respectively, of the molded article orproduct 21. A reference insert 15 and a sink insert 16 face each otheracross the cavity 17 at both sides of the mirror pieces or inserts 13and 14 and are formed with the transfer surface 15 and the non-transferor sink surface 16 a, respectively, in order to form the referencesurface or C surface 25 and the sink surface or B surface 25,respectively, of the molded article or product 21. The surfaces 13 a, 14a, 15 a, and 16 a of the inserts 13, 14, 15, and 16, respectively, formthe cavity 17. The reference surface 15 a and the non-transfer or sinksurface 16 a of the reference insert 15 and the sink insert 16,respectively, are each formed with fine irregularities or undulations62.

A gate or sprue (not shown) is formed in the mirror piece or insert 13of the stationary or upper mold 11, while a gate or sprue 63 is formedin the mirror piece or insert 14 of the movable or lower mold 12. Thegate or sprue 63 formed in the lower mirror piece or insert 14 in themovable or lower mold 12 is capable of communication with the gate orsprue (not shown) of the stationary or upper mold 11. An injectionmolding machine (not shown) injects molten resin 20 into the cavity 17via the gate or sprue (not shown) of the stationary or upper mold 11 andthe gate or sprue 63. A vent hole 18 is formed in the sink insert 16.The vent hole 18 is open to the cavity 17 at one end and connected to abore 19 at the other end. The bore 19 communicates with a flow ratecontrol unit 67 via a vent hole 18, formed in the movable or lower mold12, and a feed tube 64. The flow rate control unit 67 is connected to agas feed unit 65 via both a pressure control unit 68 and a temperaturecontrol unit 66.

The gas feed unit 65 constitutes a gas source. The temperature controlunit 66 controls the temperature of a gas, such as air 9, fed from thegas feed unit 65. The pressure control unit 68 controls the pressure ofthe gas fed from the gas feed unit 65. Further, the flow rate controlunit 67 controls the flow rate of the gas, such as air 9, fed from thegas feed unit 65. The vent hole 18 communicates with an exhaust valve 69via a vent hole 18, formed in the movable or lower mold 12, and anexhaust tube 60. The gas, such as air 9, fed from the gas feed unit 65to the vent hole 18 is either discharged to the outside, when theexhaust valve 69 is open, or is introduced into the cavity 17, when theexhaust valve 69 is closed.

In this embodiment, the flow rate control unit 67, pressure control unit68, temperature control unit 66, and gas feed unit 65 constitute afeeding device. The feeding device constitutes gas feeding means incombination with the vent holes 18, 18, 18, and 18, feed tube 64, andexhaust valve 69.

The operation of the illustrative embodiment will be described withreference to FIGS. 33A, 33B, and 34A-34C. Briefly, this embodiment ischaracterized in that a step of pressing the sink surface or B surface24 of the molded article or product 21 of molten resin 20 with the gas,e.g., air 9, is combined with at least one of a step of lowering thetemperature of the sink surface or B surface 24 of the molded article orproduct 21 below the temperature of the mirror surfaces 22 and 23 of themolded article or product 21, a step of forming a gas layer between thesink surface or B surface 24 of the molded article or product 21 and thesink insert 16, and a step of lowering the temperature of the sinkinsert 16 facing the sink surface or B surface 24 of the molded articleor product 21 below the temperature of the mirror inserts 13 and 14. Thefollowing description will concentrate on the combination of all of suchsteps.

First, the exhaust valve 69 is opened to feed a small amount of gas,e.g., air 9, to the vent hole 18 via the flow rate control unit 67,thereby cooling the sink insert 16 (see FIG. 33A). Specifically, theflow rate of the ga, e.g., air 9, is selected so as to prevent the gasfrom entering the cavity 17. Otherwise, the gas, e.g., air 9, wouldenter the cavity 17 and cool even the mirror surfaces 13 and 14. Ifdesired, the temperature of the gas may be controlled in order topromote the effective cooling of the sink insert 16.

After the mold assembly 10 has been heated to a temperature lower thanthe softening point of resin, but before molten resin 20, heated to atemperature above its softening point, is injected into the cavity 17,the flow rate unit 67 and pressure control unit 68, respectively. As aresult, the gas, e.g., air 9, is admitted into the cavity 17.Subsequently, the molten resin 20 begins to be injected into the cavity17 (see FIG. 33B). The increase in the flow rate of the gas, e.g., air9, promotes the cooling of the molten resin 20, while the increase inthe pressure of the gas allows the gas to press the sink surface or Bsurface 24 of the molded article or product 21 and the sink insert 16.

After the injection of the molten resin 20 (see FIG. 34A), the exhaustvalve 69 is closed, while the pressure of the gas, e.g., air 9, isadequately controlled by the pressure control unit 68. As a result, thecavity 17 is caused to dwell at a preselected pressure and cooled (seeFIGS. 34B and 34C). When the pressure inside the cavity 17 dropssubstantially to zero, the stationary or upper mold 11 is released fromthe movable or lower mold 12. Thereafter, the molded article or product21 is taken out of the cavity 17.

This embodiment achieves the same advantages as the fifteenthembodiment, and in addition, achieves an advantage that the sink surfaceor B surface 24 of the molded article or product 21 is constantlypressed and therefore, easily separates from the sink insert 16. Thisallows the sink surface or B surface 24 of the molded article or product21 to sink more positively.

FIG. 35 shows an alternative arrangement, wherein the flow control unit67 is connected to a gas conduit work 70 available in a factory.

Constantly pressing the sink surface or B surface 24 of the moldedarticle or product 21, as shown and described, is not essential.Alternatively, at least one of three different methods may be used, asfollows: lowering the temperature of at least one of the non-transfersurfaces of the molten resin below the temperatures of the transfersurfaces from the beginning to the end of the injection of the moltenresin; forming a gas layer between at least one of the non-transfersurfaces of the molten resin and the mold assembly; and lowering thetemperature of the mold portion facing at least one of the non-transfersurfaces of the molten resin below the temperature of the mold portionfacing the transfer surfaces.

The fifteenth to eighteenth embodiments, shown and described above, havethe following unprecedented advantages.

(1) The non-transfer surface of a molded article or product obtains aparting ability earlier than the other surfaces of the same. Thissuccessfully prevents the transfer surfaces of the molded article orproduct from sinking and thereby allows desired mirror surfaces to befaithfully transferred to the molded article or product in a shortmolding cycle.

(2) The non-transfer surface of molten resin is held to a temperaturelower than the temperature of the transfer surfaces from the end ofresin injection to the beginning of cooling. Consequently, a temperaturedifference does not occur between the transfer surfaces and thenon-transfer surfaces during cooling, so that an internal strain isprevented from remaining in the molded article or product after theopening of the mold assembly. This not only prevents the accuracy of thetransfer surfaces from decreasing, but also prevents the entire moldedarticle or product from deforming.

(3) A gas layer is formed between the non-transfer surface of the resinand the mold assembly until the cooling step begins after the injectionof the molten resin, preventing the pressure difference between thetransfer surfaces and the non-transfer surface from increasing duringcooling. Consequently, the internal strain of the molded article orproduct is prevented from remaining after the opening of the moldassembly. This not only prevents the accuracy of the transfer surfacesfrom decreasing, but also prevents the entire molded article or productfrom deforming.

(4) The gas layer is formed between the non-transfer surfaces of theresin and the mold assembly, and/or the temperature of the non-transfersurface, until the cooling step begins after the injection of the moltenresin, thereby preventing a difference in temperature or pressurebetween the transfer surface and the non-transfer surface fromincreasing during cooling. Consequently, the internal strain of themolded article or product is prevented from remaining after the openingof the mold assembly. This not only prevents the accuracy of thetransfer surfaces from decreasing, but also prevents the entire moldedarticle or product from deforming.

(5) The temperature of the non-transfer surface of the resin is lowered,the non-transfer surface is pressed, and/or the gas layer is formedbetween the non-transfer surface of the resin and the mold assembly.This allows the non-transfer surface to sink with priority by use of asimple construction.

(6) Gas feeding means can be implemented only if a vent hole is formedin the mold assembly and communicates with a feeding device. Thisprevents the configuration of the mold assembly from being complicated.

(7) By cooling the non-transfer surface of the mold assembly withcooling means, it is possible to cool the non-transfer surface of theresin. The non-transfer surface can therefore be caused to sink by aninexpensive construction of the mold assembly.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof. For example, while the embodimentshave concentrated on a molded article or product in the form of arectangular lens (having two mirror surfaces or optical surfaces), thepresent invention is similarly applicable to, e.g., a mirror having asingle mirror surface or a prism having a plurality of mirror surfaces.

What is claimed is:
 1. A mold assembly comprising: a pair of moldsincluding: an inner mold surface forming a cavity having a preselectedvolume into which a molten molding material is adapted to be injectedvia a gate to fill said cavity and thereby form a molded product havinga sink surface and at least one mirror surface after said molten moldingmaterial has cooled, said inner mold surface including: at least onetransfer surface portion against which said at least one mirror surfaceof said molded product is adapted to be formed; at least onenon-transfer surface portion against which said sink surface portion ofsaid molded product is adapted to be formed, wherein said at least onenon-transfer surface is approximately perpendicular to said at least onetransfer surface; at least one vent hole having first and second ends,said first end of said vent hole being formed in said at least onenon-transfer surface portion of said inner mold surface so as to be incommunication with said cavity; at least one bore having a first end incommunication with said second end of said vent hole so that apreselected air pressure is adapted to be applied to said molten moldingmaterial in said cavity via said bore and said vent hole, said at leastone bore having a larger diameter than said at least one vent hole; andat least one exhaust hole located so as to be adjacent to but spacedfrom said vent hole and having a first end formed in said at least onenon-transfer surface portion of said inner mold surface.
 2. The moldassembly as claimed in claim 1, wherein said exhaust hole is a singlecontinuous hole in cross-section which surrounds said vent hole.
 3. Themold assembly as claimed in claim 2, wherein an upper portion of saidexhaust hole has a rounded contour similar to a rounded contour of anupper portion of said sink surface of said molded product.
 4. The moldassembly as claimed in claim 1, wherein said exhaust hole comprises aporous member.
 5. The mold assembly as claimed in claim 1, wherein saidexhaust hole has air forcibly discharged therethrough.
 6. The moldassembly as claimed in claim 1, wherein said exhaust hole has an openingwidth ranging from approximately 0.001 mm to approximately 0.5 mm. 7.The mold assembly as claimed in claim 1, wherein said exhaust hole isactually two exhaust holes both spaced a predetermined distance from aside of said vent hole and said bore.
 8. The mold assembly as claimed inclaim 1, wherein said as least one exhaust hole is a first exhaust holeand a second exhaust hole, said first exhaust hole being locatedadjacent to, but spaced from a first side of said vent hole and saidsecond exhaust hole being located adjacent to, but spaced from a second,opposite side of said vent hole.
 9. The mold assembly as claimed inclaim 2, wherein said vent hole is rectangular in cross-section and saidbore is either an annular rectangle in cross-section or an annularapproximately semi-circular arc is cross-section.
 10. A mold assemblycomprising: a movable mold portion and a stationary mold portion; afirst mirror insert having first and second sections, said first sectionof said first mirror insert being embedded in said movable mold portionand said second section of said first mirror insert extending outwardlyfrom a surface of said movable mold portion; a second mirror inserthaving first and second sections, said first section of said secondmirror insert being embedded in said stationary mold portion and saidsecond section of said second mirror insert extending outwardly from asurface of said stationary mold portion; a reference insert and a sinkinsert each located between said movable mold portion and saidstationary mold portion, wherein together said first and second mirrorinsert, said reference insert, and said sink insert form: an inner moldsurface defining a cavity having a preselected volume into which amolten molding material is adapted to be injected via a gate to fillsaid cavity and thereby form a molded product having a sink surface andat least one mirror surface after said molten molding material hascooled, said inner mold surface including: at least one transfer surfaceportion against which said at least one mirror surface of said moldedproduct is adapted to be formed; and at least one non-transfer surfaceportion against which said sink surface portion of said molded productis adapted to be formed, wherein said at least one non-transfer surfaceis approximately perpendicular to said at least one transfer surface; atleast one vent hole having first and second ends, said first end of saidvent hole being formed in said at least one non-transfer surface portionof said inner mold surface so as to be in communication with saidcavity; at least one bore having a first end in communication with saidsecond end of said vent hole so that a preselected air pressure isadapted to be applied to said molten molding material in said cavity viasaid bore and said vent hole; and at least one exhaust hole located soas to be adjacent to but spaced from said vent hole and having a firstend formed in said at least one non-transfer surface portion of saidinner mold surface.
 11. The mold assembly as claimed in claim 10,wherein said reference insert has a surface lying adjacent to saidsecond section of said second mirror insert and said second section ofsaid first mirror insert.
 12. The mold assembly as claimed in claim 11,wherein said sink insert has a surface lying adjacent to said secondsection of said second mirror insert and said second section of saidfirst mirror insert.
 13. The mold assembly as claimed in claim 12,wherein said vent hole, said bore and said exhaust hole extend throughsaid sink insert from an outer peripheral surface of said sink insert tosaid cavity.